Mass flowmeter of the Coriolis type

Abstract

 A mass flowmeter of the Coriolis type with a tube that forms a mechanically closed loop through which a medium flows during operation, and with excitation means for causing the loop to rotate in an oscillatory mode about an axis of rotation during operation. The loop has a starting point and an end point. The starting and end points are situated close together and are connected to a flexible inlet tube and a flexible outlet tube, respectively, which extend preferably in parallel and close together. The loop is resiliently suspended from the frame of the flowmeter by means of the flexible inlet and outlet tubes, which preferably form one integral piece with the tube of the loop.

Description

[0001] The invention relates to a mass flowmeter of the Coriolis type, with a tube that forms a loop through which a medium flows during operation, and with excitation means for causing said loop to oscillate about a primary axis of rotation during operation, said axis lying in the plane of said loop.

[0002] Such a mass flowmeter is known from USP 4 658 657.

[0003] The known mass flowmeter comprises a looped tube that is not closed (half a turn) and that forms a transverse branch at one side which is connected to two lateral branches clamped in at the opposite side in a mounting beam. The latter is mounted in a support such that it can rotate about a central axis lying in the plane of the loop. An electromagnetic excitation system cooperating with the (magnetic) mounting beam provides an oscillatory rotation (vibration) of the mounting beam with the loop about the central axis. When a medium flows through the loop that rotates about the central axis, Coriolis forces are generated in the transverse branch that is oriented perpendicularly to the axis of rotation, resulting in a vibration of the loop about an axis perpendicular to the excitation axis of rotation. This vibration, which is proportional to the flow, is superimposed on the fundamental oscillation and leads to a phase shift between the oscillations performed by the ends of the transverse branch. The phase difference is proportional to the Coriolis force and accordingly to the flow.

[0004] It is a disadvantage of the known system, however, that the mounting beam used for the excitation of the loop constitutes an additional mass. This prevents a change in the excitation frequency as a function of the density of the medium flowing through the tube, with the result that a measurement of the density (an additional property of a Coriolis flowmeter) becomes less accurate.

[0005] The invention has for its object inter alia to provide a flowmeter with an excitation system that is capable of measuring the density more accurately.

[0006] The mass flowmeter of the kind mentioned in the opening paragraph is for this purpose characterized in that the loop follows a substantially circumferential, mechanically closed path, in that the loop is connected to a flexible inlet tube and to a flexible outlet tube for the flowing medium, and in that the loop is resiliently suspended from a frame by means of said flexible inlet and outlet tubes such that the suspension allows a movement about two perpendicular axes in the plane of the loop, one for the excitation movement and one for the Coriolis movement that arises when a medium is flowing through the tube.

[0007] According to the invention, the mechanically closed, loop-shaped tube is resiliently suspended by means of the inlet and the outlet tube, which together act as a flexible connecting element. That is to say: the inlet and outlet tubes are flexible and capable of torsion to a greater or lesser degree and thus act as spring elements. This suspension allows a movement about two mutually perpendicular axes that lie in the plane of the loop, one for the excitation movement and one for the Coriolis movement.

[0008] A mass flowmeter with a loop-shaped tube suspended in this manner has an enhanced sensitivity because the inlet and outlet tubes have been given freely resilient longitudinal dimensions that are as great as possible, and the suspension stiffness of the loop is a minimum for a given tube diameter, especially if said tubes extend in parallel and close to one another. An additional advantage of a fastening of the inlet and outlet tubes close to one another to the frame is that the temperature sensitivity of the flowmeter is less than in the case in which the fastening points are far removed from one another.

[0009] As will be explained in more detail further below, various versions of the inlet and outlet tubes are possible, each with its own advantages. A practical embodiment that is preferable on mechanical grounds is, for example, one in which the loop is formed as one integral piece with the inlet tube and the outlet tube.

[0010] Irrespective of the further design, it is important that the inlet and outlet tubes should be fastened to the frame with the aid of fastening means at a predetermined distance from the location where they are connected to the loop, which predetermined distance defines their free path length.

[0011] The inlet and outlet tubes may lie in the plane of the loop or outside the plane of the loop. The more they lie in the plane of the loop, however, the better. If they lie in the plane of the loop, they may lie entirely or partly within the loop. Alternatively, they may lie entirely outside the loop, for example in line in a plane transverse to the loop, in line in the plane of the loop, parallel in a plane that is at an angle to the plane of the loop, or parallel in the plane of the loop but outside the loop itself.

[0012] A preferred embodiment is characterized in that the inlet and outlet tubes extend mutually parallel over their free path lengths and closely next to one another, because their torsional stiffness is lower then, and preferably have a maximum free path length up to the fastening means, because this reduces their bending stiffness. A maximum free path length can be realized in that the inlet and outlet tubes are fastened to the frame by fastening means outside the loop.

[0013] As will be explained in more detail further below, the sensing tube can be excited in the so-termed twist or rotation mode or in the so-termed parallel or swing mode. The twist mode is preferred within the scope of the invention, in particular if it takes place about an axis of rotation lying in the plane of symmetry of the loop.

[0014] An embodiment that is advantageous on account of its sensitivity is characterized in that the loop forms a rectangle with two parallel lateral tubes, a first transverse tube connected to first ends of the lateral tubes, and two second transverse tubes connected at their one ends to the second ends of the lateral tubes and at their other ends to the inlet tube and the outlet tube, respectively.

[0015] A very compact embodiment of the above design is characterized in that the inlet and the outlet tube extend in the plane of the loop and within the loop, closely next to one another, and the inlet and the outlet tube extend on either side of an axis of symmetry of the loop and are fastened to the frame in a location closer to the first transverse tube than to the second transverse tubes. More in particular, the free path length of the inlet tube and the outlet tube amounts to at least 50% of the height of the loop as viewed in a direction parallel to the inlet and outlet tubes. This means for a looped tube having a rectangular shape that the free path length of the inlet and the outlet tube is at least 50% of the length of each of the lateral tubes.

[0016] The loop should preferably be mechanically closed. A first embodiment is for this purpose characterized in that the second transverse tubes are mechanically interconnected adjacent their connections to the inlet and the outlet tube. A second embodiment is characterized in that the inlet and the outlet tube extend mutually parallel and closely together over their free path lengths and are mechanically interconnected over at least part of their free path lengths.

[0017] The excitation (i.e. causing to vibrate) of the looped tube of the mass flowmeter according to the invention may be effected in various ways, for example by means of a magnetic disc adhered to the tube and an electromagnet with an air-spaced coil. The present loop, however, is an intrinsically very light object, and if excitation means are fastened thereto it will cost an additional amount of energy to bring the loop into resonance. It is accordingly preferred to use an excitation technique that does not require the addition of further components to the loop.

[0018] In this respect, an embodiment is characterized in that the excitation means comprise means adapted to generate an electric current in the wall of the tube and magnet means that generate a magnetic field transverse to the direction of the current in the tube wall so as to exert, through interaction with the current through the tube, electromagnetic forces (so-termed Lorentz forces) on the tube with the object of causing the tube to rotate about one of the perpendicular axes. Lorentz forces are forces that are generated when an electric current passes through a magnetic field.

[0019] In this connection, a first embodiment is characterized in that the magnet means comprise a permanently magnetic magnet yoke with one air gap through which a tube portion extends. A so-termed parallel or swing excitation mode can be generated thereby.

[0020] An embodiment for realizing a so-termed rotation or twist excitation mode is characterized in that the magnet means comprise a permanently magnetic magnet yoke with two air gaps through which respective tube portions extend, oppositely directed magnetic fields obtaining in said air gaps.

[0021] For measuring the effect of the Coriolis forces, two sensors adapted for measuring displacements of two points of the tube as a function of time are preferably arranged on either side of the primary rotation axis (the excitation axis). If there is little space, for example in the case of a tube having a delta shape, it is advantageous if the magnet yoke has a central opening between the air gaps, and the sensors are arranged in said opening.

[0022] Given a tube that forms a rectangular loop, there is more space for positioning the sensors and the yoke as favorably as possible. In an embodiment, the magnet yoke for a tube forming a rectangular loop is arranged at one side of the rectangular loop, with the sensors at the side opposite thereto.

[0023] The invention will now be explained in more detail with reference to a drawing showing a number of embodiments of the invention.

Fig. 1 is a front elevation of a Coriolis flowmeter according to the invention;

Fig. 2 is a perspective view of the flowmeter of Fig. 1;

Fig. 3 is a perspective view of the looped tube used in the flowmeter of Figs. 1 and 2;

Figs. 4A and B are front elevations and Figs. 4C to F are perspective views of alternative embodiments of the suspension of the looped tube of Fig. 3; and

Figs. 5 and 6 are front elevations of alternative embodiments of the looped tube of Fig. 3.

Description of the Figures

[0024] Fig. 1 shows a flowmeter 1 of the Coriolis type with a looped tube 2 that is bent into a rectangular shape so as to follow a substantially circumferential path (substantially one full turn), and that comprises a flexible inlet tube 3 and a flexible outlet tube 4 for a fluid medium. Preferably, the loop 2 and the inlet and outlet tubes 3, 4 are portions of one and the same tube. The tube 2 in its entirety is bent into a rectangular shape, but the corners are rounded such that it can be bent into this shape. The inlet tube 3 is connected to a supply line 6 and the outlet tube 4 to a discharge line 7 via a supply/discharge block 20. The inlet and outlet tubes 3, 4 of this embodiment extend within the loop 2 and are fastened (i.e. clamped) to a frame 13 by fastening means 12. The fastening is provided in a location such that the free path length of the inlet and the outlet tube 3, 4 (i.e. the portion of the inlet/outlet tube 3, 4 between the connection of the second transverse tube portions 2a, 2b and the location of the clamping to the fastening means 12) is at least 50%, preferably 60% of the length of each of the lateral tube portions 2c, 2d. The flexible inlet and outlet tubes 3, 4 do not form part of the loop 2 but provide a flexible fastening of the loop 2 to the frame 13. The loop 2 may thus be regarded as being flexibly suspended by means of the inlet and outlet tubes. The loop 2 and the inlet and outlet tubes 3, 4 may advantageously be manufactured from one integral piece of tubing. This may be, for example, a stainless steel tube with an outer diameter of approximately 0.7 mm and a wall thickness of approximately 0.1 mm. Depending on the outer dimensions of the loop 2 and the pressure the tube is to be able to withstand (for example 100 bar), the outer diameter of the tube will usually be smaller than 1 mm and the wall thickness 0.2 mm or less.

[0025] The looped tube 2 is shown in more detail in Fig. 3, where components known from Fig. 1 have been given the same reference numerals. The tube 2 consists of a substantially rectangular framework comprising two parallel lateral tubes 2d and 2e, a first transverse tube 2c connected to first (lower) ends of the lateral tubes 2d and 2e, and two second transverse tubes 2a and 2b connected at one side to second (upper) ends of the lateral tubes and at the other side to the centrally returning inlet and outlet tubes 3 and 4, respectively. The rectangular loop 2 preferably has rounded corners. The tubes 3 and 4, which run closely together on either side of and symmetrically with respect to the main axis of symmetry S of the loop 2, are fastened to the fastening means 12, for example by clamping or soldering or welding, said means 12 in their turn being fastened to the base plate 13. Fig. 3 shows by way of example a cavity 14 in fastening means (block) 12, in which the tubes 3, 4 are retained. The inlet and outlet tubes 3, 4 are flexible and act as it were as a suspension spring for the loop 2. This suspension allows a motion of the loop 2 both about the main axis of symmetry S and about a second axis S' situated in the plane of the loop 2 and perpendicular to the main axis of symmetry S.

[0026] To close the loop 2 mechanically (i.e. to interconnect the beginning and end of the loop mechanically, directly or indirectly), the tubes 3, 4 are preferably connected to one another along the extent of their free path lengths, for example in that they are welded or soldered together. Fig. 3 shows a few connection spots with reference numeral 15 by way of example.

[0027] An alternative is that the transverse tubes 2a and 2b are connected to one another and possibly to the inlet and outlet tubes 3 and 4, for example by fastening to a support element 16 in a location where they come close together. The connection between the second transverse tubes 2a, 2b and/or between the inlet and outlet tubes 3, 4 is important for creating a mechanically closed loop so as to obtain the correct vibration modes in operation.

[0028] To obtain a good spring action, the tubes 3, 4 preferably have as great as possible a free path length d. More in particular, d is preferably greater than 0.5 times the length D of the lateral tubes 2d and 2e. The fastening means 12 are accordingly positioned closer to the first transverse tube 2c than to the second transverse tubes 2a, 2b.

[0029] The inlet tube 3 and the outlet tube 4 are bent out of the plane of the loop 2 beyond the fastening means 12 in the embodiment of Fig. 3, i.e. they curve around the first transverse tube 2c so as to be connected to supply and discharge lines. They preferably move away from one another in that direction to facilitate this connection. This is more clearly seen in Fig. 2.

[0030] Fig. 4 shows a number of alternatives for arranging and clamping the inlet and outlet tubes, which alternatives all utilize the suspension principle presented with reference to Fig. 3. An additional slackness is added to the fastening of the loop in the alternatives of Fig. 4 in that the fastening points are located farther away.

[0031] Fig. 4A shows a substantially rectangular looped tube 21 with inlet and outlet tubes 20, 21 that extend within and in the plane of the loop, said tubes moving away laterally from a given point in different directions. The tubes 20, 21 are clamped in (fixedly retained) in fixation points 22, 23 that lie within the loop. The fact that the fixation points are laid farther away than in the situation of Fig. 3 provides the resilient suspension with an additional slackness. The tubes 20, 21 may continue beyond the clamping points 22, 23, passing with slight curves over the lateral tubes 24, 25 of the looped tube 21, or they may be perpendicularly curved backwards.

[0032] Fig. 4B shows a substantially rectangular looped tube 26 with parallel inlet and outlet tubes 27, 28 extending in the plane of the looped tube and continuing from a given point with an extra bend to their respective fastening points. The tubes 27, 28 are clamped in at locations 29 and 30. The fastening points are located even farther away in this manner than in Fig. 4A.

[0033] It may be desired to increase the free path lengths of the inlet and outlet tubes to beyond the lowermost transverse tube (cf. Fig. 2) by bending them out of the plane of the loop (over the lowermost transverse tube), but this would be to the detriment of the spring characteristic. This problem is solved by the construction of Fig. 4C.

[0034] Fig. 4C shows a substantially rectangular looped tube 31 with inlet and outlet tubes 32, 33 extending in one plane closely next to one another. The tubes 32, 33 are clamped in by fastening means 34 situated outside the loop 31. The tubes 32, 33 have no kinks because the lower transverse tube 35 of the loop 31 has been curved outwards.

[0035] Fig. 4D shows a substantially rectangular looped tube 36 with inlet and outlet tubes 37, 38 extending first in the plane of the loop 36 and then separating so as to move away from the plane of the loop 36 in mutually opposed directions. The tubes 37, 38 are clamped in at a given distance from the plane of the loop 36 in locations 39 and 40 in order to increase the free path length.

[0036] Fig. 4E presents an alternative to the construction of Fig. 4D and shows a substantially rectangular looped tube 41 with inlet and outlet tubes 27, 28 not extending initially in the plane of the loop, as was the case in Fig. 4D, but splitting up and moving away immediately from the plane of the loop 41 in mutually opposed directions. The tubes 42, 43 are clamped in in locations 44 and 45 outside the plane of the loop 41.

[0037] Fig. 4F shows a substantially rectangular looped tube 46 with inlet and outlet tubes 47, 48 that move away immediately from the plane of the loop 46 in the same direction. The inlet and outlet tubes 47, 48, which are preferably parallel, are clamped in in a location 49 outside the plane of the loop 46. This construction forms an alternative to the construction of Fig. 4E and provides a greater stiffness, in particular if the tubes 47, 48 are mechanically interconnected.

[0038] It is advantageous in those embodiments in which the clamping spots lie outside the plane of the loop (Figs. 4D, 4E, 4F) if they are arranged symmetrically with respect to the plane of the loop.

[0039] The embodiments described above show a rectangular loop. It is alternatively possible, however, to use shapes other than the rectangular one, as long as the loop forms a (substantially) closed turn. Some of these alternatives, all resiliently suspended by means of inlet and outlet tubes situated within the loop, are shown in front elevation in Fig. 5.

[0040] Fig.5A is a front elevation of a polygonal loop 50 (octagonal in this case, but six sides or more than eight sides are also possible).

[0041] Fig. 5B shows an elliptical loop 51.

[0042] Fig. 5C shows a diamond-shaped loop 52.

[0043] Fig. 5D shows a trapeziform loop 53.

[0044] The loop shapes shown in Fig. 5 have the resilient portions of their inlet and outlet tubes entirely within the loop. However, they may alternatively lie partly outside the loop, as in Figs. 4C and 4D, or entirely outside the loop, as in Figs. 4E and 4F. The sensitivities of the mass flowmeters having the loop shapes shown in Fig. 5 differ only slightly. The rectangular shape of Fig. 3, however, provides the greatest sensitivity for given external dimensions.

[0045] Figs. 6A and 6B show looped tubes 54 and 61 according to the invention arranged into delta shapes that may be regarded as modifications of the rectangular tube of Fig. 3. The looped tube 54 (Fig. 6A) has an inlet tube 55 that is connected to the loop 54 adjacent a starting point 56 of the loop 54. The incoming flow traverses a first oblique side of the delta shape from this starting point 56, then the base 60, and finally a second oblique side of the delta shape. At the end point 58, the delta-shaped tube 54 is connected to an outlet tube 57. The inlet and outlet tubes 55, 57 extend in parallel, close together in the plane of the loop 54 and within the loop 54 and are fastened to a frame (not shown) by fastening means 59. A resilient suspension of the loop 54 is thus realized comparable to that of the loop 2 in Fig. 3. The tube 54 may be provided with projections or "ears" at the transitions between the base 60 and each of the oblique sides.

[0046] The same holds mutatis mutandis for the looped tube 61 shown in Fig. 6B, which is also bent into a delta shape. Here the inlet and outlet tubes 62, 64 extend in parallel, close together in the plane of the loop 61 but outside the loop 61, so that the free path length of the spring formed by the inlet and outlet tubes 62, 64 can be greater than in the construction of Fig. 6A. The inlet and outlet tubes 62, 64 are connected via fastening means 66 to a frame (not shown) at one side and to the starting point 63 and end point 65 of the delta-shaped tube 61 at the other side.

[0047] To obtain a mechanical closure of the looped tubes 54 and 61 shown in Figs. 6A and 6B, a mechanical connection may be made between the starting points 56, 63 and the end points 58, 65 of the respective loops 54, 61. An alternative is to connect the respective inlet and outlet tubes 55, 57 and 62, 64 mechanically over at least part of their free path lengths, for example by means of welding or soldering. The mechanical interconnection suppresses the occurrence of natural frequencies that could interfere with the measurement.

[0048] The closed delta-shaped tubes according to the invention may alternatively be constructed with a double loop instead of a single loop. Depending on the design, it is possible to use the same directions of flow as well as mutually opposed directions of flow in the two loop portions.

[0049] The excitation (into oscillation) of the looped tube of the mass flowmeter according to the invention may take place in various manners, for example by means of a magnetic disc adhered to the tube and an electromagnet with an air-spaced coil. The present loop, however, is of itself a very light object, and if excitation means are fastened thereto, it will require an additional amount of energy to bring the loop into resonance. Therefore, Figs. 1 and 2 show special excitation means that render it unnecessary to add further components to the loop.

[0050] In the construction of Figs. 1 and 2, in both of which the same reference numerals are used, the excitation means for causing the loop 2 to oscillate about the main axis of symmetry S (the primary or excitation axis of rotation) comprise a permanent magnet yoke 8 fastened to the frame 13, said yoke having two gaps 9 and 10 through which portions 2a and 2b (denoted the second transverse tubes above) of the looped tube 2 are passed, as well as means for introducing an electric current into the tube 2. These are means for inducing a current in the tube 2 in the present case.

[0051] The current is induced in the tube by means of two transformer cores 17, 17a provided with respective coils 18a, 18b, through which cores the respective tube portions 2c and 2d are passed. The combination of the magnetic fields generated in the gaps 9 and 10 of the permanently magnetic yoke 8, which fields are transverse to the direction of the current and are oppositely directed, and an (alternating) current induced in the tube 2 exerts a torque on the tube owing to which it starts to oscillate or rotate about the axis S (in the so-termed twist mode). When a medium flows through the tube, the tube will start to rotate about an axis S' transverse to the axis S (in the so-termed swing mode) under the influence of Coriolis forces. During operation the (sinusoidal) displacements of points of the tube portion 2c, which are representative of the flow, are detected by means of a first sensor 11 a and a second sensor 11 b, and optionally a third sensor 11 c. The first and the second sensor are arranged on either side of the first axis of rotation S. A third sensor 11 c may serve for correction purposes. The sensors may be, for example, of an electromagnetic, inductive, capacitive, or ultrasonic type. In the present case, however, optical sensors were chosen. The sensors 11 a, 11 b, and 11c (Figs. 1 and 2) each comprise a U-shaped housing that is fastened to the frame13, with a light source (for example an LED) placed in the one leg and a photosensitive cell (for example a phototransistor) opposite the light source in the other leg. The lateral tube 2c is capable of moving between the legs of the U-shaped sensor housings 11a and 11b (and 11c, if present).

[0052] In brief, the invention relates to a mass flowmeter of the Coriolis type with a tube that forms a closed loop through which a medium flows during operation and with excitation means for causing the loop to rotate in an oscillatory mode about an axis of rotation during operation. The loop has a starting point and an end point. The starting and end points are situated close together and are connected to a flexible inlet tube and a flexible outlet tube, respectively, which extend preferably in parallel and close together. The loop is resiliently suspended from the frame of the flowmeter by means of the flexible inlet and outlet tubes, which preferably form one integral piece with the tube of the loop.

Claims

1. A mass flowmeter of the Coriolis type, with a tube that forms a loop through which a medium flows during operation, and with excitation means for causing said loop to vibrate about an axis of rotation during operation, said axis lying in the plane of said loop,
characterized in that the loop follows a substantially round, mechanically closed path, in that the loop is connected to a flexible inlet tube and to a flexible outlet tube for the flowing medium, and in that the loop is resiliently suspended from a frame by means of said flexible inlet and outlet tubes such that the resulting suspension allows a movement about two perpendicular axes in the plane of the loop, one for the excitation movement and one for the Coriolis movement that arises when a medium is flowing through the tube.

2. A mass flowmeter as claimed in claim 1,
characterized in that the loop is formed as one integral piece with the inlet tube and the outlet tube.

3. A mass flowmeter as claimed in claim 1,
characterized in that the inlet tube and the outlet tube are fastened to the frame by fastening means at a predetermined distance from the location where they are connected to the loop, which predetermined distance defines their free path length.

4. A mass flowmeter as claimed in claim 3,
characterized in that the inlet tube and the outlet tube extend with their free path lengths in parallel and closely next to one another and are fastened to the fastening means next to one another.

5. A mass flowmeter as claimed in claim 1, wherein the loop forms a rectangle with two parallel lateral tubes, a first transverse tube connected to first ends of the lateral tubes, and two second transverse tubes connected at their one ends to the second ends of the lateral tubes and at their other ends to the inlet tube and the outlet tube, respectively.

6. A mass flowmeter as claimed in claim 5,
characterized in that the inlet tube and the outlet tube extend in the plane of the loop and within the loop, closely next to one another on either side of an axis of symmetry of the loop, and are fastened to the frame in a location closer to the first transverse tube than to the second transverse tubes.

7. A mass flowmeter as claimed in claim 1,
characterized in that the excitation means excite the loop in a twist excitation mode.

8. A mass flowmeter as claimed in claim 3,
characterized in that the free path length of the inlet and the outlet tube amounts to at least 50% of the height of the loop as viewed in a direction parallel to the inlet and outlet tubes.

9. A mass flowmeter as claimed in claim 5,
characterized in that the free path length of the inlet and the outlet tube is at least 50% of the length of each of the lateral tubes.

10. A mass flowmeter as claimed in claim 5,
characterized in that the second transverse tubes are mechanically interconnected adjacent their connections to the inlet and the outlet tube.

11. A mass flowmeter as claimed in claim 1 or 2,
characterized in that the inlet and the outlet tube extend mutually parallel and closely together over their free path lengths and are mechanically interconnected over at least part of their free path lengths.

12. A mass flowmeter as claimed in claim 1,
characterized in that the excitation means comprise means adapted to generate an electric current in the wall of the tube and magnet means that generate a magnetic field transverse to the direction of the current in the wall of the tube so as to exert Lorentz forces on the tube through interaction with the current through the tube.

13. A mass flowmeter as claimed in claim 12,
characterized in that the magnet means comprise a permanently magnetic magnet yoke with one air gap through which a tube portion extends.

14. A mass flowmeter as claimed in claim 12,
characterized in that the magnet means comprise a permanently magnetic magnet yoke with two air gaps through which respective tube portions extend, oppositely directed magnetic fields obtaining in said air gaps.

15. A mass flowmeter as claimed in claim 1 and 13,
characterized in that two sensors for measuring displacements of two points of the tube as a function of time are arranged on either side of the primary axis of rotation.

16. A mass flowmeter as claimed in claim 14,
characterized in that the magnet yoke has a central opening between the air gaps, and in that the sensors are arranged in said opening.

17. A mass flowmeter as claimed in claim 12,
characterized in that the tube forms a rectangular loop, the magnet yoke is arranged at one side of the rectangular loop, and the sensors are arranged at the side opposite thereto.

Drawing